LOW EMISSION AND LOW EXCESS AIR SYSTEM

Abstract: A pulverized coal steam generator (10) employing low NOx firing methods is operated at very low excess air levels. This is possible because the unburned carbon in the flyash is measured and the pulverizers (24) are adjusted to control the particles size of the pulverized coal and maintain a desired carbon level.

Description: LOW EMISSION AND LOW EXCESS AIR SYSTEM Background of the Invention The present invention relates to a coal fired steam generating system and method which produces low emissions of nitrogen oxides and employs low excess air. Nitrogen monoxide (NO) and nitrogen dioxide (NO2) are by-products of the combustion process of virtually all fossil fuels. Historically, the quantity of these inorganic compounds in the products of combustion was not sufficient to affect boiler performance and their was presence was largely ignored. In recent years, oxides of nitrogen have been shown to be key constituents in the complex photochemical oxidant reaction with sunlight to form smog. Today, the emission of NO2 and NO (collectively referred to as NOx) is regulated by both state and federal authorities and has become an important consideration in the design of fuel firing equipment. The formation of NOx in the combustion process is often explained in terms of the source of nitrogen required for the reaction. The NOx can originate from the oxidation of nitrogen in atmospheric air in which the product is referred to as "thermal NOx'' or from the organically bound nitrogen components found in all fossil fuels which are termed "fuel NO,". The formation of thermal NOx can be decreased by reducing the time, temperature, and concentration of 02. On the other hand, the fuel NOx is not very temperature dependant but is a strong function of the fuel-air stoichiometry and residence time. A number of techniques to control fuel NOx have been developed that involve modification of the combustion process such as low excess air firing and air staging. Under fuel-rich conditions and with sufficient residence time available, the conversion of fuel nitrogen to harmless molecular nitrogen, rather than to NOx can be maximized. One of the developments that has been used to reduce the formation of NOx is the offset air or concentric firing technique disclosed in U.S. Patent 4,294,178. In this firing technique, tangential firing is employed with the fuel and primary combustion air being introduced tangentially to an imaginary circle in the center of the furnace and with the secondary combustion air being directed tangentially to a larger concentric circle. This patent also discloses the use of flue gas recirculation which is also tangentially introduced between the fuel and secondary air streams. This concentric or offset air firing technique has the effect of reducing the formation of NOx while simultaneously reducing the slagging and corrosion of the furnace walls. As indicated, another technique for reducing the formation of NOx is the use air staging or overfire air. The overfire air nozzles are located in the windbox of the uppermost coal nozzles. Approximately 20% of the total combustion air to a burning zone is introduced through these overfire air nozzles. As a result, the fireball is at slightly sub-stoichiometric air conditions. When combined with low excess air firing in the range of perhaps 15 to .tY% excess air, the NOx formation is controlled by driving the major fraction of the fuel nitrogen compounds into the gas phase under overall fuel-rich conditions. In this atmosphere of oxygen deficiency, there occurs a maximum rate of decay of the evolved intermediate nitrogen compounds to N2. Following the introduction of the remaining overt ire air, the slow burning rate reduces the peak flame temperature to curtail the thermal NOx production in the later stages of combustion. The use of even lower levels of excess air (below 15%) would further reduce the formation of NOx and increase plant efficiency but that has not been practical in the past because of the resulting incomplete combustion of the fuel and the high levels of unburned carbon in the flyash. Summarv of the Invention A steam generator employing one or more low NOx firing methods for coal is operated at further reduced excess air levels while controlling the carbon loss in the flyash. More specifically, the excess air levels are reduced to reduce NOx emissions and increase efficiency while controlling the particle size of the coal to minimize carbon loss all in conjunction with the adjustment between the secondary and overfire air to minimize NOx formation. Brief Description of the Drawings Figure 1 is a diagrammatic representation of a coal fired steam generator in the nature of a vertical sectional view. Figure 2 is a sectional plan view of the furnace section of the steam generator taken along line 2-2 of Figure 1. Figure 3 is a diagrammatic front view of one of the tangential firing units. Figure 4 is a graph of the percent carbon in the flyash versus the percent excess air as a function of the particle size of the coal. Figure 5 is a representation of the various parameters measured and the functions controlled. Description of the Preferred Embodiments Figure 1 of the drawings illustrates a typical steam generating unit 10 having a furnace section 12, a horizontal gas pass 14 and a back pass 16. The furnace section is lined with water wall tubes 18 in which the steam is generated. The horizontal gas pass and the back pass contain various combinations of economizers, superheaters and reheaters which are all conventional for such steam generators and have not been specifically identified in the drawings. The steam generator illustrated is of the known tangentially fired type. The coal silo 20 feeds coal to the feeder 22 which controls the rate of flow to pulverizer 24. These pulverizers not only have means for pulverizing but also include adjustable classifiers which control the particle size of the coal discharged from the pulverizer. The hot primary combustion air is also fed to the pulverizer by duct 25 and it carries the pulverized coal through and out of the pulverizer to the burners. With proper adjustment of the classifier, the particles of the proper size are discharged with the primary combustion air and the oversize particles are recycled to the pulverizing rollers. Pulverizers of this type are conventional and the details have not been illustrated. The pulverized and sized coal particles together with the primary combustion air are fed through the coal pipes 26 to the coal nozzles 28 in the tangential windboxes 30. As shown in Figure 3, each windbox has a plurality of coal nozzles 28 plus a plurality of secondary air nozzles 32. The windboxes are connected to each other by the air plenums 34 as seen in Figure 2. The air preheater 36, which transfers the heat from the combustion gases to the incoming air, supplies the air for both the primary air to the pulverizers through duct 25 and the secondary air to the plenum 34 and windboxes 30 through the duct 38. Located between the plenum 34 and the windboxes 30 are dampers at 40 which control the quantity of air fed into the furnace from the windboxes at any particle level of the windboxes. As seen in Figure 2, concentric firing is employed in which the secondary air is directed away from the fuel towards the adjacent furnace wall in order to reduce the entrainment of secondary air by the expanding primary air/coal fire ball. The coal and primary air are directed at the tangent of the small circle 42 along lines 44 while the -secondary air is directed along lines 46 tangent to the larger circle 48. Thus, air is effectively withheld from the fire ball and effects the early furnace stoichiometry reducing the formation of NOx Also, the air being directed along the walls of the furnace helps prevent slagging and corrosion. The ability to maintain an oxygen concentration at the wall while having a deficiency of oxygen in the fireball is critical to the success of low excess air operation. Figure 3 is a simplified illustration of a tangential firing windbox showing the dampers 40, the coal/primary air nozzles 28 and the secondary air nozzles 32. At the top of the windbox are the overfire air nozzles 50 which are controlled by the dampers 52 also at the top. In the illustrated version of the tangential windbox, the fuel/primary air nozzles have been grouped or clustered together (rather than alternating with the secondary air) which is another way of controlling the rate of burning and thus the temperature and NOx production. In accordance with the present invention, one object is to perform the combustion process with low excess air, below 15% and preferably between 5 and 10% as compared with a normal excess air rate of 20% or more. As previously explained, a mere reduction in the excess air will result in unburned fuel which will appear as carbon in the flyash. In order to accomplish low excess air firing, the present invention controls the combustion process according to the quantity of carbon in the flyash. A number of commercial instruments are available for this purpose. One technique is to burn the flyash sample turning the carbon to carbon dioxide and then measuring the quantity of carbon dioxide given off by a known quantity of flyash. Carbon content can also be measured by resistivity and neutron activation techniques. The flyash sample is preferably taken in the flue gas stream leaving the back pass of the steam generator or leaving the air preheater. An alternative location would be in the flyash hopper of the precipitator. Shown in Figure 1 is a flyash carbon detector 54 located in the back pass of the steam generator 10 following the back pass heat exchange surfaces. The measurement signal from the detector 54 is fed to a control unit 56 which is adapted to control the classifier of the pulverizer 24 to control the particle size of the coal. It might be assumed that the pulverizer classifier could merely be operated at the finest setting so that it always provides very fine particles to keep the carbon down. However, it is undesirable to operate the pulverizes with the particle size setting less than needed for the circumstances. First of all, operating the pulverizers at a particle size less than necessary takes considerable energy and this energy requirement must be weighed against the benefits to be derived. Also, if the classifier is set too fine, there is increased recirculation of the larger particles from the classifier to the pulverizer rolls which in turn reduces the capacity of the pulverizer to process fresh coal. This results in inadequate pulverizer capacity for the steam generator or the requirement for excessive pulverizer capacity. The carbon detector 54 is connected through a plant operating controller to the pulverizer 24 so as to control the pulverizer classifier settings. The graph of Figure 4 illustrates the relationship between excess air and the carbon in the flyash as a function of the particle size of the pulverized coal. It can readily be seen that the percent carbon in the flyash increases as the excess air is reduced and that it decreases as the particle size is reduced. It can also be seen that the percent carbon in the flyash can be maintained at a desired level even when the excess air is reduced if the particle size is also reduced. If the flyash is to be utilized in byproducts such as cinder block or aggregate, no more than 5% carbon in the flyash is merely sent for disposal, a tradeoff occurs between the energy lost in the carbon in the flyash and the energy required to pulverize the coal finer. In such instances, a plant efficiency analysis is useful. These computerized systems take plant data and calculate the plant efficiency on-line. The maximum plant efficiency would then determine the required carbon in the flyash. One such system is the available Combustion Engineering Total On-Line Performance System (CETOPS). Figure 5 is a schematic representation of the pertinent operating parameters that would be measured and the corresponding function to be controlled. In this system, certain standard control linkages are maintained. The fuel flow is still maintained by the steam drum pressure as a measure of load and the total air flow is maintained by oxygen measurement in the flue gas. However, in the present invention, the oxygen setpoint is reduced to achieve the desired low amount of excess air. The NOx production as measured in the flue gases is used to control the ratio of overfire air compared to secondary air.

Claims: Claims: 1. A method of operating a pulverized coal fired steam generator comprising the steps of: a) pulverizing coal and firing said pulverized coal in said steam generator; b) maintaining the excess combustion air at a level below 15% above stoichiometric; c) measuring the percentage of carbon in the flyash; d) establishing a desired percentage carbon in the flyash; e) adjusting the particle size of said pulverized coal to maintain said desired percentage carbon in the flyash. 2. A method as recited in claim 1 wherein said percentage carbon in said flyash is maintained at 5% or less. 3. A method as recited in claim 1 wherein said excess combustion air is maint ined at a level between 5 and 10% above stoichiometric.

Inventors: BOZZUTO CARL R, BOZZUTO, CARL, R.
Applicants: COMBUSTION ENG, COMBUSTION ENGINEERING, INC.
Application year: 1994
Main classification: F23K1
IPC codes: F23C5, F23C99, F23K1, F23N1, F23N5